1. Technical Field
The present invention relates to a driving circuit, a liquid crystal device, an electronic apparatus, and a method of driving the liquid crystal device.
2. Related Art
A liquid crystal device that displays an image using liquid crystal is known. Such a liquid crystal device, for example, includes a liquid crystal panel and a backlight arranged to be opposite the liquid crystal panel.
The liquid crystal panel includes a pair of substrates and liquid crystal interposed between the pair of substrates.
The liquid crystal panel includes a plurality of scanning lines and a plurality of capacitance lines alternately provided at every predetermined interval, and data lines crossing the plurality of scanning lines and the plurality of capacitance lines and being provided at every predetermined interval.
Pixels are provided at intersections of the scanning lines and the data lines. Each pixel includes a pixel capacitor having a pixel electrode and a common electrode, a thin film transistor (hereinafter, referred to as a TFT), and a storage capacitor of which one electrode is connected to the capacitance line and the other electrode is connected to the pixel electrode. The plurality of pixels are arranged in a matrix to form a display area.
A gate of the TFT is connected to the corresponding scanning line, a TFT source is connected to the corresponding data line, and a TFT drain is connected to the corresponding pixel electrode and the other corresponding electrode of the storage capacitor.
In the above-described liquid crystal panel, a scanning line driving circuit connected to the plurality of scanning lines, a data line driving circuit connected to the plurality of data lines, and a capacitance line driving circuit connected to the plurality of capacitance lines are provided.
The scanning line driving circuit sequentially supplies a selection voltage for selecting a scanning line to the plurality of scanning lines. For example, when supplying the selection voltage to any scanning line, the TFT connected to the corresponding scanning line is turned on and the pixel related to the corresponding scanning line is selected.
The data line driving circuit supplies an image signal to the plurality of data lines when the scanning lines are selected. An image voltage based on the image signal is applied to the pixel electrodes through TFTs in the ON state.
The data line driving circuit supplies the data lines with the image signal of which the voltage (hereinafter, referred to as a positive polarity) is higher than that of the common electrode and applies the image voltage based on the image signal of the positive polarity to the pixel electrodes. The data line driving circuit supplies the data lines with the image signal of which the voltage (hereinafter, referred to as a negative polarity) is lower than that of the common electrode and applies the image voltage based on the image signal of the negative polarity to the pixel electrodes. At this time, the data line driving circuit alternately performs application of a positive polarity voltage and application of a negative polarity voltage at every one horizontally scanning period.
The capacitance line driving circuit supplies a predetermined voltage to the capacitance lines.
The above-described liquid crystal device operates as follows.
The selection voltage is sequentially supplied to the scanning lines to turn TFTs connected to the scanning lines to the ON state and all of the pixels related to the scanning lines are selected. In addition, in synchronization with the selection of the pixels, the image signal is supplied to the data lines. Accordingly, the image signal is supplied to all the selected pixels through TFTs in the ON state and the image voltage based on the image signal is applied to the pixel electrodes.
When the image voltage is applied to the pixel electrodes, a potential difference between the pixel electrodes and the common electrodes induces a driving voltage to be applied to the liquid crystal. When the driving voltage is applied to the liquid crystal, alignment or order of molecules of the liquid crystal is changed, light transmitted through the liquid crystal from a backlight is changed, and a gray scale level is displayed.
The driving voltage is applied to the liquid crystal for an interval three orders of magnitude greater than the interval of time for which the image voltage is applied by the storage capacitors.
The above-described liquid crystal device is used for, for example, a portable apparatus. However, there has been recently a demand for reducing the power consumption of portable apparatuses. Accordingly, there has been suggested a liquid crystal device capable of having reduced power consumption by applying the image voltage to the pixel electrodes, and subsequently turning TFTs to an OFF state and changing the voltage of the capacitance lines (for example, see JP-A-2002-196358).
An operation of the liquid crystal device of the known example that changes the voltage of the capacitance lines in the manner described in JP-A-2002-196358 will be described with reference to FIGS. 13 and 14.
FIG. 13 is a timing chart illustrating an application of the positive polarity in the liquid crystal of the known example. FIG. 14 is a timing chart illustrating an application of the negative polarity in the liquid crystal of the known example.
For example, the liquid crystal device of the known example has scanning lines and capacitance lines of 320 rows and the data lines of 240 columns.
In FIGS. 13 and 14, GATE(j) denotes a voltage of the scanning line of a j-th row (where j is an integer satisfying 1≦j≦320) and VST(j) denotes a voltage of the scanning line of the j-th row. SOURCE(k) denotes a voltage of the data line of a k-th row (where k is an integer satisfying 1≦k≦240). PIX(j, K) denotes a voltage of the pixel electrode of a pixel in the j-th row and the k-th column corresponding to an intersection of the j-th scanning line and the k-th data line. VCOM denotes a voltage of the common electrode commonly provided to each pixel.
First, an operation of application of the positive polarity in the liquid crystal device of the known example will be described with reference to FIG. 13.
When the data line driving circuit supplies the selection voltage to the j-th scanning line at time t31, the voltage GATE(j) of the j-th scanning line increases, and thus becomes a voltage VGH at time t32. In this way, TFTs connected to the j-th scanning line all turn on.
When the data line driving circuit supplies the positive image signal to the k-th data line at time t33, the voltage SOURCE(k) of the k-th data line increases, and thus becomes a voltage VP8 at time t34.
The voltage SOURCE(k) of the k-th data line that is the image voltage based on the positive image signal is applied to the image electrode of the pixel in the j-th row and the k-th column through the ON state TFT connected to the j-th scanning line. For this reason, a voltage PIX(j, k) of the pixel electrode of the pixel in the j-th row and the k-th column increases, and thus becomes the voltage VP8 at time t34, which is the same as the voltage SOURCE (k) of the k-th data line.
When the scanning line driving circuit stops supplying the selection voltage to the j-th scanning line at time t35, the voltage GATE(j) of the j-th scanning line decreases, and thus becomes the voltage VGL at time t36. In this way, TFTs connected to the j-th scanning line all enter the OFF state.
When the capacitance line driving circuit supplies a predetermined voltage to the j-th capacitance line at time t36, a voltage VST(j) of the j-th capacitance line increases, and thus becomes a voltage VSTH at time t37.
When the voltage VST(j) of the j-th capacitance line increases, charges corresponding to the increased voltage are distributed to the storage capacitors and the pixel capacitors in all pixels related to the j-th capacitance line. For this reason, the voltage PIX(j, k) of the pixel electrode of the pixel in the j-th row and the k-th column increases, and thus becomes a voltage VP9 at time t37.
That is, in the liquid crystal device of the known example, when the positive polarity is applied, the image voltage based on the image signal of the positive polarity is applied to the pixel electrodes, and then the voltage of the capacitance lines is increased. At this time, the voltage of the pixel electrodes increases by as much as a sum of a voltage increased by the charges corresponding to the voltage increased by the image voltage and the increased voltage of the capacitance lines, referring to the voltage of the common electrodes.
Next, an operation of application of the negative polarity in the liquid crystal device of the known example will be described with reference to FIG. 14.
When the scanning line driving circuit supplies the selection voltage to the j-th scanning line at time t41, the voltage GATE(j) of the j-th scanning line increases, and thus becomes the voltage VGH at time t42. In this way, TFTs connected to the j-th scanning line all turn on.
When the data line driving circuit supplies the image signal of the negative polarity to the k-th data line at time t43, the voltage SOURCE(k) of the k-th data line decreases, and thus becomes a voltage VP11 at time t44.
The voltage SOURCE(k) of the k-th data line that is the image voltage based on the image signal of the negative polarity is applied to the image electrode of the pixel in the-j row and the k-th column through the ON state TFT connected to the j-th scanning line. For this reason, the voltage PIX(j, k) of the pixel electrode of the pixel in the j-th row and the k-th column decreases, and thus becomes a voltage VP11 at time t44, which is the same as the voltage SOURCE(k) of the k-th data line.
When the scanning line driving circuit stops supplying the selection voltage to the j-th scanning line at time t45, the voltage GATE(j) of the j-th scanning line decreases, and thus becomes a voltage VGL at time t46. In this way, TFTs connected to the j-th scanning line all turn off.
When the capacitance line driving circuit supplies a predetermined voltage to the j-th capacitance line at time t46, the voltage VST(j) of the j-th capacitance line decreases, and thus becomes a voltage VSTL at time t47.
When voltage VST(j) of the j-th capacitance line decreases, charges corresponding to the decreased voltage are distributed to the storage capacitors and the pixel capacitors in all pixels related to the j-th capacitance line. For this reason, the voltage PIX(j, k) of the pixel electrode of the pixel in the j-th row and the k-th column decreases, and thus becomes a voltage VP10 at time t47.
That is, in the liquid crystal device of the known example, when the negative polarity is applied, the image voltage based on the image signal of the negative polarity is applied to the pixel electrodes, and then the voltage of the capacitance lines is increased. At this time, the voltage of the pixel electrodes increases by as much as a sum of a voltage decreased by the charges corresponding to the voltage decreased by the image voltage and the decreased voltage of the capacitance lines, referring to the voltage of the common electrodes.
In the liquid crystal device as described in the known example, even when an amplitude of the image voltage is reduced, a potential difference between the voltage of the common electrodes and the voltage of the pixel electrodes can be increased by applying the image voltage to the image electrodes and changing the voltage of the capacitance lines. As a result, a display quality can be prevented from being deteriorated by guaranteeing the amplitude of the driving voltage applied to the liquid crystal and the consumption power can be reduced by reducing the amplitude of the image voltage.
In the liquid crystal device as described above in the known example, the voltage of the capacitance lines is changed and the charges are moved between the storage capacitors and the pixel capacitors to change the voltage of the pixel electrodes. For this reason, when irregularity in characteristics occurs among the storage capacitors, an amount of the charges moving between the storage capacitors and the pixel capacitors is affected. Even when the same image voltage is applied to the pixel electrodes, the irregularities can happen in the voltages of the pixel electrodes. Accordingly, irregularities can happen in a gray scale level of the pixels, thereby deteriorating the display quality.
Further, in the liquid crystal device as described in the known example, since the voltage of the capacitance lines is changed to be different from that of the pixel electrodes or the common electrodes, one electrode of the storage capacitors connected to the capacitance lines is required to be separately formed from the pixel electrodes or the common electrodes. For this reason, in liquid crystal devices using modes such as In-Plane Switching (IPS) and Fringe-Field Switching (FFS) in which the pixel electrodes and the common electrodes constituting the pixel capacitors are provided on one substrate of a pair of substrates with liquid crystal interposed therebetween and the pixel capacitors and the storage capacitors are incorporated, it is difficult to form the liquid crystal device as described in the above-described in the known example.